Sodium Thiosulphate + Hydrochloric Acid Coursework

Plan

Aim

I must produce a piece of coursework investigating the rates of reaction when Sodium Thiosulphate + Hydrochloric Acid are mixed.

The reaction that will be used is:

Na S O + 2CHl 2Na Cl + S + SO + H O

Sodium + Hydrochloric Sodium + Sulphur + Sulphur + Water Thioslphte Acid Chloride Dioxide

Variables

Other variables, which may affect the outcome of this experiment, if not kept constant, are as follows:

Temperature of liquids. I believe that the hotter the temperature, the faster the rate of reaction. This is because when a substance is heated up, each particle has more energy, resulting in more collisions between the free moving particles in a liquid, causing more particles to react, therefore increasing the overall rate of reaction. The second effect this has is that the particles collide with more force, increasing the chance of the particles reacting. I will keep this variable constant so that it does not affect the results by avoiding heating the sodium thiosulphate in any way, and so it will stay at room temperature.
Human Error/People. This experiment relays on people and their eye sight, because to determine how long the reaction takes a cross is put under the container and when it isn’t disappears the timer is stopped and the time is taken down. People have different eyesight’s and reaction times that can change the out comes.
Container. Although this does not affect the rate of reaction, in that it doesn't affect the chemical reaction itself, but what this variable does affect is our measurement of how long the reaction has taken to complete. I will be judging the time taken for the reaction to finish as the time it takes for the reacted mixture of sodium thiosulphate to go completely cloudy so that I cannot see a cross drawn underneath the flask. If the flask is wider then the cloudiness will appear less concentrated, because it will be spread across the larger area of the bottom of the conical flask, and so it will take longer for the cross to disappear, and the results will increase. I will keep this variable constant by using the same conical flask for each experiment.
Cross. The material I use draw the cross underneath the conical flask, which I use in determining when the reaction has finished, could affect how easily the cross is seen. This in turn could affect the results because if the cross is seen more easily, if it was drawn with a thick marker, our measurement of how long the reaction took to complete would be longer and so the rate of reaction would seem slower. Equally, if the cross was hard to see, then it would disappear more quickly and the rate of reaction would seem quicker. I will keep this constant by using the computer to design crosses which are the same size and thickness.
Concentration of acid. The more concentrated the hydrochloric acid, the faster the reaction will be, because there will be more particles in the same volume.
Stirring/ mixing. If I shake the conical flask while it is reacting, then the reaction may speed up and therefore the rate of reaction would be quicker. Likewise, if I stir it then it will increase the rate of reaction, because there will be a greater chance of the particles colliding with other particles which they can react with because the two types of particles will be more mixed together and also. I will prevent this from affecting the results by not stirring or shaking the mixture in any way.

Prediction

The concentration will increase the rate of reaction because if the concentration is higher, then there will be more molecules of Hydrochloric Acid in a given volume, and so it is reasonable to say that it greatly increases the chances of one of those molecules colliding with the hydrochloric acid molecules. This means that more of the molecules will be colliding and reacting within a given time, and so the rate of the reaction has speeded up. However, I do not have enough information on the subject to be able to make a quantitative prediction at this time.

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Prediction

Method

Firstly, I will dilute the Hydrochloric Acid by way of ratios (see diagram 1) I will measure 10cm³ of solution each time, with a burette, but I will vary the volume of Hydrochloric Acid compared to the volume of water diluting it, making different concentrations of Hydrochloric Acid. I have found from preliminary tests that these concentrations react at rates which I can measure quite accurately with the equipment I have. Then, I will take a cross which I have ready made on the computer, and place it underneath a flask/container. Into the conical flask, I will pour the 10cm³ of sodium thiosulphate solution and 10cm³ of hydrochloric acid, and start the stop clock as soon as all the solution has been added. When the mixture has become opaque, or when the cross underneath the flask isn’t visible any longer, I will stop the clock and record how long it has taken to reach this stage. I will repeat this 3 times with each concentration, making sure that the conical flask is fully washed out, because I will be using the same flask every time, so that the flask does not introduce any new variables into the results. To find a rate of reaction from the results I will collect, I will divide 1 by the time in seconds. This provides me with directly comparable rates of reaction, because as the time gets longer, the rate is much slower. I will repeat the whole experiment twice, in order to reduce the effects of any mistaken results that may occur, by using averages to make a conclusion.

Diagram 1

To make the experiment safe, I will be wearing goggles at all times, in case something unexpected occurs i.e. solution may split and end up in my eyes. I will be washing my hands regularly and thoroughly washing the equipment before each reaction to make sure that it is not only a fair test, but that it does not cause something dangerous to happen. I will also not drink or eat throughout this experiment as the solution may splash on the food or drink. And finally I will not sit down when conducting this experiment because if some liquid or apparatus may happen to fall on me I wont be able to move out of the way.

Fair Test

In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the Hydrochloric Acid and thiosulphate will not be mixed up. The amount of Hydrochloric Acid will be 10 cm3 each time expect for when diluting (see diagram1), and the amount of thiosulphate will be fixed at 10 cm3. Also I will be the only person looking at the cross.

Using my preliminary experiments I decided on using the following apparatus:

1 beaker
1 burette
1 stand
1 cramp
1 conical flask
1 stopwatch
1 pair of goggles
1 apron

Results and Evidence

The results which I have collected, without any changes to the planned method, are shown above. I have also plotted them onto a graph (see below) so that the trend can be seen more easily.

Conclusion

The results, and the graph appear to show roughly what I predicted. As the concentration of sodium thiosulphate is increased, then the rate of reaction increases also. What I believe to be happening is that as the concentration decreases, there are less Hydrochloric Acid molecules around in the reacting mixture, meaning that it will take longer for each hydrochloric acid molecule to ‘find’ a sodium thiosulphate for it to react with. Overall, this decreases the rate of reaction because it takes longer for the mixture as a whole to react fully, because of the extra time it takes for the molecules to collide with each other.

If the molecules move around in the liquids at the same speed each time (because they have a constant temperature), then the further they have to travel to reach another molecule, the longer it will take for them to reach that molecule and then react.

Evaluation

The results of this experiment appear to be quite accurate and so I can make a definite conclusion. From this, I can speculate that the experiment was conducted well, and that the results have only been affected by other variables by little or no amount. However, looking at the graph, I can see that there is a result that do not follow the general trend, and show a higher rate of reaction than would be expected at that point. I believe that perhaps this was caused by a change in another variable I remember I had wash out the flash with hot water and so the partials had more energy and collide more to speed up the reaction time.

I think that the only flaws in the experiment were the washing the flash with hot water and in the judging of when the reaction had completed.

If I was to repeat the experiment, I would find a completely different way of measuring when the reaction has completed. Perhaps some sort of electronic device could detect turbidity more precisely. This device could shine a bright light, perhaps a laser through the chemicals, to be received by a very sensitive LED on the other side. This LED could be connected to a stop clock which would stop once the LED stopped receiving direct light (it would still be able to detect some light, but it would not be direct because it would be diffused by the turbidity of the chemicals).